Variable capacitors are advantageous as different electronic responses can be obtained by variation of the capacitance. The structures presently used to implement variable or tunable capacitors, however, have significant performance and practical limitations. Movable parallel plates, while providing variable capacitance for radio tuning, are bulky, lossy, noisy, generally operate over only a limited range of frequencies, or have any number of these limitations. A “lossy” component or device has a high insertion loss (IL), which is the ratio of power dissipated in the component to power delivered to a load. An electronic varactor is a semiconductor device that adjusts capacitance responsive to an applied voltage. Varactors are typically lossy and noisy, and are therefore generally ineffective for high-frequency applications, particularly those above 200 MHz. Hence, they are not suited for tuning insertion loss-critical devices such as filters and multiplexers in wireless applications, particularly where Code Division Multiple Access (CDMA) is used. Another implementation providing variable capacitance is a micro-electro-mechanical system (MEMS). This is a miniature switching device that physically selects a different capacitor responsive to an applied signal. MEMS, however, is typically costly, unreliable, requires a substantial control voltage, and enables only a discrete set of pre-selected capacitance values.
Because of their variable dielectric constant, ferroelectric materials are good candidates for making tunable capacitors or other tunable components. Under presently used measurement and characterization techniques, however, tunable ferroelectric components have gained the reputation of being consistently and substantially lossy, regardless of the processing, doping or other fabrication techniques used to improve their loss properties. They have therefore not been widely used. Ferroelectric tunable components operating in RF or microwave regions are perceived as being particularly lossy. This observation is supported by experience in RADAR applications where, for example, high RF or microwave loss is the conventional rule for bulk (thickness greater than about 1.0 mm) f-e materials especially when maximum tuning is desired. In general, most f-e materials are lossy unless steps are taken to improve (reduce) their loss. Such steps include, but are not limited to: (1) pre and post deposition annealing or both to compensate for O2 vacancies, (2) use of buffer layers to reduce surfaces stresses, (3) alloying or buffering with other materials and (4) selective doping.
As demand for limited range tuning of lower power components has increased in recent years, the interest in ferroelectric materials has turned to the use of thin film rather than bulk materials. The assumption of high ferroelectric loss, however, has carried over into thin film work as well. Conventional broadband measurement techniques have bolstered the assumption that tunable ferroelectric components, whether bulk or thin film, have substantial loss.
A broadband measurement of the capacitance value of a ferroelectric capacitor is typically obtained using a device such as an LRC meter, impedance analyzer or a network analyzer. From power measurements, one can calculate the lossiness of the capacitor. The inverse of lossiness is referred to as the Quality Factor (“Q”). Thus, a lossy device will have a low Q and a more efficient device will have a high Q. Q measurements for ferroelectric capacitors with capacitances in the range of about 0.5 pF to 1.0 pF operating in a frequency range of 1.8 GHz to 2.0 GHz, obtained using conventional measurement techniques, are typically claimed to be in the range of 10-50. This is unacceptably inefficient, and ferroelectric tunable components are therefore considered undesirable for widespread use. In wireless communications, for example, a Q of greater than 80, and preferably greater than 180, and more preferably greater than 350, is necessary at frequencies of about 2 GHz.
As will be shown below, conventional ferroelectric components have been wrongly fabricated, measured and characterized. As a result, it is commonly assumed that ferroelectric tunable components are very lossy with Qs in the range of 10-50 in the L-band. Ferroelectric tunable devices operating in other frequency bands have also been labeled as having Qs unacceptable for most applications.